Rocky Mountain spotted fever (RMSF) due to tick-transmitted Rickettsia rickettsii is a serious infectious disease prevalent in the Americas. R. conorii, the etiologic agent of Mediterranean spotted fever phylogenetically related to R. rickettsii, can also cause debilitating human infections associated with significant morbidity and mortality. Pathogenic rickettsiae preferentially target microvascular endothelium of small- and medium-sized vessels resulting in vascular inflammation and compromised permeability, collectively termed as `rickettsial vasculitis'. As obligate intracellular pathogens, efficient adhesion and invasion into host cells followed by quick escape into nutrient-rich cytoplasmic niche is critical for rickettsial growth, replication, and dissemination. Through a combination of complementary approaches, we have recently discovered Fibroblast Growth Factor Receptor 1 (FGFR1) as one of the receptors utilized by rickettsiae to gain entry into host endothelial cells. Further evidence from our ongoing work clearly suggests that inhibition of FGFR1 diminishes the levels of intracellular infection in vitro and pulmonary rickettsial burden in vivo. Mechanistically, negative regulation of two host-specific microRNAs (miR424 and miR503) results in augmented expression of FGFR1 to facilitate interactions between the host cell and invading rickettsiae. The overall objective of this application is to exploit this critical `initia' interaction of pathogenic rickettsiae with the target host cells, the blocking of which prevents entry and access to intracytoplasmic environment conducive to their growth and replication, for the development of new therapeutic strategies and identification of potentially novel biomarkers for early diagnosis. Accordingly, we propose to investigate the contributions of and mechanisms underlying host miR-mediated regulation of FGF2/FGFR1 expression in the invasion of human and mouse microvascular endothelium by SF rickettsiae in vitro [Aim 1] . Specifically, we will determine the effects of miR424/miR503 mimics and antagomirs as well as FGFR1-specific inhibitors on FGF2/FGFR1 expression and rickettsial invasion and elucidate the mechanisms underlying FGFR1-mediated entry. We will next delineate the contributions of FGF2/FGFR1 interactions in the initiation and progression of RMSF in established mouse models and explore the potential for FGFR1 inhibitors and miR424/503 mimics as novel therapeutics and circulating miRs as early biomarkers of disease [Aim 2] . The multi-disciplinary approaches to accomplish these aims will not only involve physiologically relevant human and murine cell culture systems, established mouse models of infection closely recapitulating human disease, and cutting-edge applications of cellular and molecular transcriptomics, but will also benefit from endothelial-specific disruption of FGFR1 in mice and strong institutional expertise in all aspects of Rickettsiology research. This proposal is highly relevant to the mission of the NIH, as it aims to uncover novel targets to develop new therapeutics and identify new tools for the diagnosis of severe and potentially fatal rickettsioses afflicting human-kind across the globe.